This invention relates to well logging of earth boreholes and, more particularly, to a method and apparatus for determining permeability of earth formations surrounding a borehole.
It is well known that during the drilling of an earth borehole, such as an oil or gas well, fluid in the drilling mud begins invading the surrounding formations, due to the difference in pressure between the mud in the well bore and the formation fluids. The extent of invasion depends upon various factors, including the nature of the mud cake formed on the well bore, and the porosity and permeability of the surrounding formations. Invasion by the mud filtrate in a given region of the well bore will typically continue for a period of time after the region has been drilled. Characteristics of the "invaded zone" are important in the determination of formation characteristics, and various types of well logging equipment can provide measurements of invaded zone parameters that are useful in formation evaluation.
Typically, the invasion profile around the well bore is considered to be generally cylindrical in shape. For example, in a particular region (or bed) of the formation, the invaded zone is normally visualized as a cylindrical annulus having a thickness that can vary from a fraction of an inch to several feet.
Although conventional well logging models assume a cylindrical invasion profile, it was recognized decades ago that the actual profile of invading fluid in a formation bed can be substantially affected by gravity. H. G. Doll noted that in salt-water bearing sands of high permeability, it was often observed that the depth of invasion by mud filtrate is quite small near the bottom boundaries of the beds, whereas the top parts of the beds are deeply penetrated. (H. G. Doll, "Filtrate Invasion In Highly Permeable Sands", H. G. Doll, The Petroleum Engineer, 1955). He indicated that this observation was brought to light by resistivity logs, in which the shallow investigation log gives the same very low readings as the deep investigation log over the lower part of the beds, and an appreciably higher reading over the upper part. This observation suggested to Doll that, in this kind of formation, after the mud filtrate has been forced through the mud cake, it does not flow horizontally into the formation, but has a tendency to rise, since it is often less saline (and therefore less dense) than the interstitial water originally located in the pores of the invaded formations. As a consequence of the difference in densities, the mud filtrate is subject to an ascensional force, and its velocity is the resultant of two components: one vertical (upward) component, caused by the ascensional force, and one horizontal (radial) component, due to the rate of filtration. Doll described the nature of the invasion front as it advances, with time, into the formation, and set forth a number of relationships representing the radial and vertical movement and positions of the advancing filtrate.
Although Doll's work laid a basic foundation for understanding vertical movement of the invading filtrate, succeeding years have produced little, if any, development on this basic foundation that has resulted in new and commercially useful techniques in the logging art for evaluating earth formations.
Determination of formation permeability, including the vertical and horizontal components thereof, is important in evaluating the behavior and producibility of a formation. However, it is generally considered difficult to obtain accurate permeability information, particularly from conventional types of logging measurements.
It is among the objects of the present invention to provide improved logging techniques and apparatus for evaluating earth formations surrounding a borehole to determine characteristics that are affected by the gravity-induced movement and location of invading fluids. It is also among the objects of the invention to provide improved technique and apparatus for determining permeability of earth formations.